Very massive stars, pair-instability supernovae and intermediate-mass black holes with the SEVN code

Abstract: Understanding the link between massive ($\gtrsim 30$ M${\odot{}}$) stellar black holes (BHs) and their progenitor stars is a crucial step to interpret observations of gravitational-wave events. In this paper, we discuss the final fate of very massive stars (VMSs), with zero-age main sequence (ZAMS) mass $>150$ M${\odot}$, accounting for pulsational pair-instability supernovae (PPISNe) and for pair-instability supernovae (PISNe). We describe an updated version of our population synthesis code SEVN, in which we added stellar evolution tracks for VMSs with ZAMS mass up to $350 $M${\odot{}}$ and we included analytical prescriptions for PPISNe and PISNe. We use the new version of SEVN to study the BH mass spectrum at different metallicity $Z$, ranging from $Z=2.0\times 10^{-4}$ to $Z=2.0\times 10^{-2}$. The main effect of PPISNe and PISNe is to favour the formation of BHs in the mass range of the first gravitational-wave event (GW150914), while they prevent the formation of remnants with mass 60 - 120 M${\odot{}}$. In particular, we find that PPISNe significantly enhance mass loss of metal-poor ($Z\leq 2.0\times 10^{-3}$) stars with ZAMS mass $60\leq $M${\mathrm{ZAMS}}/$M${\odot{}}\leq 125$. In contrast, PISNe become effective only for moderately metal-poor ($Z<8.0\times 10^{-3}$) VMSs. VMSs with M${\rm ZAMS}\gtrsim{}220$ M$\odot$ and $Z<10^{-3}$ do not undergo PISNe and form intermediate-mass BHs (IMBHs, with mass $\gtrsim 200 $M$_{\odot{}}$) via direct collapse.

Insights into Very Massive Stars, Pair-Instability Supernovae, and Intermediate-Mass Black Holes

The paper under review explores the evolutionary pathways of very massive stars (VMSs) with particular emphasis on the formation of massive black holes (BHs) via pair-instability supernovae (PISNe) and pulsational pair-instability supernovae (PPISNe), using the updated Stellar Evolution and N-body (SEVN) code. SEVN incorporates advanced stellar evolution tracks and detailed analytical prescriptions for PISNe and PPISNe, allowing for an accurate analysis of BH mass spectrum across different metallicities.

The research critically evaluates the evolutionary fate of VMSs at zero-age main sequence (ZAMS) mass exceeding 150 M$_\odot$, extending up to 350 M$_\odot$. The study focuses on two crucial processes: PPISNe, which significantly enhance mass loss in metal-poor stars, and PISNe, effective for moderately metal-poor VMSs, which are potent enough to disrupt the entire star, thereby preventing BH formation.

A key outcome of this investigation is the delineation of BH mass spectra at varying metallicities, demonstrating that PPISNe contribute to the formation of BHs with masses aligning with observations from gravitational-wave events such as GW150914 while inhibiting the formation of remnants with masses between 60 and 120 M$_\odot$. Notably, the study reveals that VMSs with ZAMS masses over 220 M$_\odot$ and metallicity below 10^{-3}</sup>donotundergoPISNeandcandirectlycollapseinto<ahref="https://www.emergentmind.com/topics/intermediate−mass−black−holes−imbhs"title=""rel="nofollow"data−turbo="false"class="assistant−link"x−datax−tooltip.raw="">intermediate−massblackholes</a>(IMBHs).</p><p>ThemethodologyentailedathoroughupdateoftheSEVNcode,incorporatingnewnon−linearinterpolationtechniquesthatenhanceprecision.Thepopulationsynthesissimulations,delineatedbyarangeofmetallicitiesfrom2.0×10⁻⁴ do not undergo PISNe and can directly collapse into <a href="https://www.emergentmind.com/topics/intermediate-mass-black-holes-imbhs" title="" rel="nofollow" data-turbo="false" class="assistant-link" x-data x-tooltip.raw="">intermediate-mass black holes</a> (IMBHs).</p> <p>The methodology entailed a thorough update of the SEVN code, incorporating new non-linear interpolation techniques that enhance precision. The population synthesis simulations, delineated by a range of metallicities from 2.0 × 10<sup>{-4}to 2.0 × 10^{-2}$, provide a robust dataset elucidating the transition from star to compact remnant under varying cosmic conditions. This analytic framework is strengthened by fitting formulae based on Woosley's models for PPISNe and PISNe, integrating empirical insights with simulation data to map stellar evolutionary outcomes across diverse metallic environments.

The implications of this work extend both practically and theoretically in the field of astrophysical research. On a practical level, it affords astronomers a refined toolset for predicting BH formation in varying galactic habitats, crucial for the interpretation of gravitational-wave detections and the identification of potential IMBH sites. Theoretically, it advances our understanding of stellar life cycles by elucidating the complex interplay of mass, composition, and evolutionary mechanisms in stellar demise and black hole formation.

Future research can be anticipated to refine these models further, integrating more sophisticated hydrodynamical simulations and extending to broader ranges of initial conditions. There remains a significant scope for studying the role of environmental factors, such as galactic dynamics and binary interactions, in shaping the outcomes predicted by singular stellar evolution models. As gravitational-wave astronomy continues to evolve, the insights from this study will be pivotal in guiding observational strategies and interpreting new cosmic events involving massive stellar remnants.